Mexico is facing a major crisis. No, not just the gangs and drug cartels so prevalent in President Trump’s fearmongering. Mexico’s water is running out — and much of the water that remains is toxic.

In its most recent report on the “state of [Mexico’s] water resources,” the Mexican government’s National Water Commission noted that water availability per inhabitant was 3,692 square meters in 2015, down from 18,053 in 1950. Of that, groundwater alone accounts for nearly 40 percent of all water use. And, in turn, of that 40 percent, 60 percent is either contaminated or over-pumped, or both.

But these conservative statistics alone do not convey the human tragedy underlying them. Like so many other people, there is Maria, a mother who lives in a poor Mexico City neighborhood without running water. She relies on expensive water deliveries, brought by truck two times per week, to fill up buckets for basic household use. Or Gloria Villanueva Rodríguez, a middle-aged woman badly sickened from tainted groundwater in a rural region of Guanajuato state. Or the hundreds of people killed, and thousands more poisoned, in the town of Salto near Guadalajara, Mexico’s second-largest city, from water in the Río Santiago — all because government authorities did virtually nothing to prevent industrial waste from being deliberately dumped into it.

The irony is that Mexico has, dating to its 1917 constitution, some of the best water laws and regulations in the world. They just aren’t enforced. That’s because powerful political and economic interests, both in the U.S. and Mexico, are deeply invested in the business of controlling Mexico’s water. As a result, they’ve crippled the laws and created a crisis that now threatens the public health of both Mexican and U.S. citizens.

A product of the great 1910 Mexican Revolution, the constitution mandated the distribution and conservation of water (and land) for all Mexicans. It placed most surface-water resources under federal jurisdiction so the government could regulate and protect them nationwide. In the following decades, the federal legislature periodically passed national water laws and even amended the constitution to update regulations as circumstances changed.

This created a regulatory regime vastly different from the one in the United States. The Environmental Protection Agency has had the authority to regulate groundwater for drinking nationwide since 2006, but the states have been charged with tackling regulations on groundwater for agriculture, by far the biggest water consumer. And they have been slow to do so. It was only four years ago that California, the largest food-producing state, passed a law mandating groundwater regulation, and only after years of withering drought, exacerbated by climate change, had resulted in massive over-pumping of groundwater.

But despite these differences, water infrastructure has brought the two countries together in a variety of ways. For instance, the U.S.-Mexico Water Treaty of 1944, signed after painstaking negotiations, mandated an equitable distribution of common water sources. It led to much cooperation through the jointly run International Boundary and Water Commission, which has included periodic updates to the treaty. (The most recent in 2012 set aside some of the Colorado River flow in the United States to restore its ecologically degraded delta in Mexico.)

Yet historically, technology, not treaties, has been the source of binational cooperation, especially as more technical solutions to water control in the U.S. have inspired water engineers in Mexico. Emulating their U.S. counterparts, especially during the New Deal in the 1930s and 1940s, Mexican engineers began to build thousands of dams. But while dams conserve water by containing it in reservoirs for human use, they also damage fragile riverine ecosystems and can forcibly displace people from their homes.

The Mexican government understood this negative side effect of dams. But its engineers were so enamored by U.S. hydraulic technology that they did very little to change course. That was certainly the case for engineer Marte R. Gómez, who served as Mexico’s Secretary of Agriculture from 1940 to 1946. He helped usher in the Green Revolution — the adoption of higher-yielding American hybrid crop seeds along with chemical pesticides and fertilizers.

Gómez called these changes the “modernization” of Mexico’s agriculture. But that modernization hinged on one essential input: water. Damming rivers, however, could not make enough of it available. Gómez thus encouraged Mexican farmers to pump groundwater profligately — even though he and other engineers knew that it would have harmful long-term environmental and public health consequences.

Seemingly insatiable demand from a rapidly growing economy and population is one major reason the Mexican government chose not to exercise its extraordinary regulatory authority over water use. But it wasn’t the only one. Some engineers had personal business interests in the very industries they were charged with regulating (the proverbial fox guarding the henhouse). Gómez, for one, quietly worked while secretary of agriculture to found his own groundwater pump-manufacturing company, Worthington de Mexico, a subsidiary of the U.S.-based multinational firm.

After he left office in 1946, Worthington headquarters in New York lobbied the U.S. government to extend low-interest loans to its new Mexican subsidiary. For its part, the Mexican government also chipped in with various financial incentives. Thanks to this generosity from both governments, Gómez profited handsomely from groundwater overuse for decades — the very overuse he had signed off on as secretary of agriculture.

Long in the making, Mexico’s water crisis has not only adversely affected millions of Mexicans. Hundreds of thousands of Americans living along the border must deal with contaminated water from binational rivers that either serve as the border (Rio Grande), flow across it (Rios Colorado and Tijuana) or else percolate under it (groundwater).

The Mexican government is not the only one contributing to this crisis. The United States government has also played a role by neglecting to invest in border water infrastructure, a violation of the U.S.-Mexico Water Treaty of 1944. And neither government has been able — or willing — to ensure that millions of border residents, currently lacking adequate and good quality water, are able to acquire it.

Although he inherited this shared water crisis, Trump has made it worse by refusing to allocate even the woefully insufficient $10 million already budgeted to the EPA to help alleviate it. Instead, he proposes spending billions on a wall that can never stop water overuse and contamination, even though the water crisis impacts far more people on the U.S. side of the border than the threat of gangs and drugs crossing over from Mexico ever have.